In wireless communication systems such as Code Division Multiple Access (CDMA) environments, it is desirable to maintain the energy used per bit as compared to the noise in a given bandwidth (E.sub.b /N.sub.o) at a level where the signal is received sufficiently well (e.g. has sufficient quality) at the subscriber unit. However, while raising the E.sub.b /N.sub.o level would provide a high quality call (e.g. by causing the base station to increase its transmit power allocated to a subscriber), it would also reduce the system capacity since the signal (E.sub.b) of one call within the bandwidth is, in general, interference (N.sub.o) to other calls. The CDMA base transmission uses orthogonal codes assigned to each user to reduce the interference on the forward link by partially separating the signals of the different users to allow operation on the same frequency with a minimum of interference within the same cell, but interference from other cells does not benefit from this coding method and is added directly as noise power (N.sub.o) Therefore, due to the interference concern, it is also desirable to keep the E.sub.b /N.sub.o level as low as possible while still providing a suitable quality signal. A more detailed description of the power level/capacity trade-off is made in related application Ser. No. 07/783,751, which is incorporated herein by reference. Since a CDMA base station uses a single bandwidth for transmitting to multiple subscriber units, codes are used to separate the signals that are sent simultaneously to each subscriber unit. For this reason, the power that is sent to each subscriber can be allocated from a maximum base transmit power, to supply the needs of each subscriber. This allows a more efficient use of the power available at the base to support the power needs of all the subscribers in the cell. As subscribers need more or less power, the allocation to each subscriber can change within limits, such that there will be a minimum and maximum that can be supplied to each subscriber. This allocation process is well known in the art, and can be considered the same as having a fixed power control range with a minimum and maximum.
In order to accomplish the objective of giving each user the minimum amount of signal to satisfy the required quality level, present proposals use a power control loop to set the E.sub.b /N.sub.o to a desired level based on the Word Error Rate (WER). When the subscriber is stopped, the E.sub.b /N.sub.o at the subscribers receiver, is gradually reduced to a level that is lower by several decibel (dB) than when the subscriber is moving. A higher E.sub.b /N.sub.o is necessary for a moving vehicle to maintain the WER in a propagation environment which is more hostile, i.e. is subject to Rayleigh fading of the signal due to the movement of the user through a large number of standing waves and reflections which produce large variations in the envelope and phase of the received signal. The WER is measured to determine power adjustments to be transmitted to the base station. These adjustments serve to maintain a nominal E.sub.b /N.sub.o level. In practice, the base transmit power that is transmitted to a user is gradually reduced which results in a reduced E.sub.b /N.sub.o and increased WER. Once the WER exceeds a certain limit, the subscriber sends a message to the base causing the base transmit power to be raised to a level where the WER at the subscriber unit is acceptable. The process then repeats. This process is referred to as a power control loop.
However, as the subscriber unit begins moving the WER will increase at a rate beyond which the low level E.sub.b /N.sub.o can be maintained. Once the WER increases past a preset point (threshold), or accelerates at a given rate, the system will gradually increase the E.sub.b /N.sub.o level (i.e. by increasing the base transmit power) to a higher level required at higher speeds.
There are many power control circuits and algorithms that are well known in the art. Almost all of these use an average signal value to determine the amount of power to send, this average being an estimate of the local mean of the signal at the receiver. The reason that the average is used rather than the instantaneous signal strength is that the bandwidth required to track the instantaneous signal would be prohibitive. Therefore, only slowly changing variations are tracked by the power control circuits, which require the receivers to be able to tolerate the fast variations of the Rayleigh fading, sometimes called fast fading. In the case of a static channel when there is no motion, the receiver operates at the lowest level of signal since it does not have to tolerate fast variations in the envelope. The power control tracks the slow changing variations, called shadow fading or Log Normal fading, by averaging out the fast fading variations to get a local mean estimate of the average signal at the receiver. The local mean estimate is generally obtained from averaging over at least 20 to 40 wavelengths of the signal so that a sufficient number of samples of the fading envelope are included in the average; alternatively a median of the samples can be used rather than an average, but the result is still a local mean estimate. Normally at least 20 samples are averaged to get a single local mean estimate which from the statistics of a Rayleigh fading envelope with uncorrelated samples would give an RMS error for the estimate of 1.0 dB compared to the true local mean.
With the Rayleigh fading averaged out, a sample of the average power can be used to set the power control. This is done now at a slow rate which is compatible with a limited bandwidth signal. The slow fading that is corrected for by the power control takes seconds to change significantly, and this is the type of fading that is compensated for by the prior art power control circuits.
A problem with the present situation as it applies to the CDMA radio system is the delay in shifting from one E.sub.b /N.sub.o level to another as the modem requirements change caused by differing channel conditions. This type of delay can result in one of two related and undesirable events which occur when the subscriber changes speed or stops. A change in speed results in a change in the required E.sub.b /N.sub.o ; which effects the quality of the channel due to the delays in the power control loop. The first event is that, from a signal perspective, the E.sub.b /N.sub.o is now less than what is considered good for the system. This causes a poorer acceptable signal quality in the channel which directly effects the user signal. The other event is that, from a system perspective, the E.sub.b /N.sub.o is now greater than what is required causing increased noise in the bandwidth for other subscribers. The former case in which the users signal is degraded is of concern since many bad frames will be received by the subscriber before the standard power control loop can adjust the signal level.
Therefore, it would be desirable to reduce the transition time for the user who is receiving bad frames due to power control loop delay to as short a time period as possible to avoid these signal outages and thus improve the overall quality of the users service.